This invention relates to a process for converting a heavy hydrocarbon oil containing asphaltenes and heavy metals in large quantities (hereinafter referred to as a "heavy oil") into a substantially asphaltene-free and heavy metal-free oil (hereinafter referred to as an "asphaltene-metal-free oil").
The heavy oils to be treated according to this invention are petroleum crude oils, residues obtained by distilling crude oil under atmospheric or reduced pressure, crude oils extracted from tar sands or mixtures thereof. These contain large quantities of high molecular weight hydrocarbon compounds having structures consisting of several fragments of condensed aromatics and connecting paraffic chains and/or naphthenic fragments (usually called asphaltenes), heavy metals, sulfur compounds and nitrogen compounds. By the term "asphaltene" as used herein is meant n-heptane insolubles that are determined by the I.P. (Institute Petroleum Great Britain) method.
The heavy oils to be treated according to this invention are those which contain asphaltenes and vanadium in large quantities. Examples of these are: (1) Venezuelan crude oil of 1.004 specific gravity (D.sub.15 /4.degree. C.) containing as high as 11.8% by weight of asphaltenes, 1240 ppm of vanadium, 5.36% by weight of sulfur, and 5800 ppm of nitrogen; (2) topped crude of Middle-Near East of 0.987 specific gravity (D.sub.15 /4.degree. C.) containing about 6.5% by weight of asphaltene, 95 ppm of vanadium, 4.45% by weight of sulfur and 3000 ppm of nitrogen; and (3) vacuum residue from the other crude oils of Middle-Near East of 1.038 specific gravity (D.sub.15 /4.degree. C.) containing about 8.2% by weight of asphaltenes, 270 ppm of vanadium, 3.53% by weight of sulfur and 7300 ppm of nitrogen, and the like.
As described above, these heavy oils contain extremely large quantities of contaminants such as sulfur and nitrogen compounds, and organometallic compounds of vanadium or nickel, etc. These contaminants are concentrated in the fraction of high molecular hydrocarbons like asphaltenes, making the catalytic hydrotreating seriously difficult. At present, therefore, asphaltenes are first separated from the feed oil by a physical process such as a solvent deasphalting process, and the deasphalted oil is hydrotreated, thus avoiding the problems with the contaminants. In removing a large amount of asphaltenes from a heavy oil by a solvent deasphalting process using a low molecular hydrocarbon such as propane, butane, pentane or the like, the asphaltene-containing fraction produced as a byproduct reaches 10-20% by weight and, in some cases, as high as more than 30% by weight, depending upon the quality of extracted oil obtained by deasphalting. Therefore, this process is not a preferred technique for treating a heavy oil containing asphaltenes.
Asphaltene is generally believed to comprise large molecules formed by association of several high molecular compounds comprising condensed aromatic rings. The asphaltene is colloidally dispersed in the oil and usually contains about 4-8% by weight of sulfur and 500-7000 ppm of heavy metals like vanadium.
Heavy oils containing such asphaltenes in a large quantity are abundantly present in nature and are regarded as promising hydrocarbon resources in the future. At present, however, they are utilized merely as an extremely low grade fuel oil or as asphalt for road paving.
With the above described background in mind, extensive investigations have been carried out on techniques for converting a heavy oil containing asphaltenes in large amounts into a valuable asphaltene-heavy metal-free oil.
At present, as an industrial process which can provide said asphaltene-free heavy metal-free oil of superior quality by hydrotreating these heavy oils, there have been proposed the following two processes:
One is a process wherein a heavy oil is subjected to catalytic hydrocracking in the presence of a catalyst having metal compound(s) supported on a carrier, and the other is a process wherein a heavy oil is subjected to catalytic hydrotreating in the presence of a catalyst consisting of non-supported metal compound(s).
In the former process, the reaction system is usually of a fixed bed or an ebullating bed type.
As the process in particular relates to this invention, there are two techniques of treatment that are disclosed in U.S. Pat. No. 2,559,285 and Japanese Open Disclosure of Patent Application No. 32003/1977 (Equivalent to U.S. Pat. No. 4,062,758) in which it has been proposed to recycle a part of the liquid reaction products separated as a heavy fraction. In these processes, however, the presence of asphaltenes and heavy metals in the charge stock would cause many economic disadvantages, which can be fully understood by those skilled in the petroleum refining technology.
That is, the asphaltenes colloidally dispersed in the charge stock consists of huge molecules that can hardly approach the active sites in pores of the catalyst. Therefore, the hydrocracking is seriously inhibited. In addition, the presence of asphaltenes increases the formation of coke and carbonaceous materials, which lead to rapid reduction of catalyst activity.
Another serious problem is the presence of significant amounts of metals in the charge stock. They accumulate on the surface of the catalyst, exert poisoning action on the catalyst and seriously shorten the catalyst life.
As is described above, when a heavy oil is treated according to the conventional catalytic hydrotreating process, the amount of catalyst consumption per unit volume of oil treated becomes exceedingly large. Furthermore, even if the above-described defects were obviated, the conventional catalysts would require severe reaction conditions for the purpose of selective asphaltene-cracking to obtain a light oil, and the reduction of the catalyst activity would be still further accelerated. In addition to the above, there also occurs rapid gas build-up due to the secondary decomposition of the cracked oil; hence, the light oil fraction cannot be obtained in a high yield and the hydrogen consumption increases. Thus, this conventional process has serious drawbacks from the standpoint of economy.
As to the latter process, U.S. Pat. No. 3,723,294 suggests a method for overcoming these difficulties. In this process, a heavy oil is hydrotreated in a slurried state with catalyst to remove metals therefrom. The resulting product is separated into a light oil fraction and a heavy oil fraction slurried with catalyst, which latter fraction is then recycled to the preceding reaction step. This process, however, appears to present serious difficulties due to the use of a slurried colloidal mixture of oil and catalyst. In general, the procedures become seriously complicated as compared with a fixed-bed process or the like. Smooth transportation of the slurry-state reactants and reaction products is difficult under high temperature and pressure. A heat exchanger for heating and cooling the slurry-state reactants and reaction products shows lower heat-exchanging efficiency in comparison with a slurry-free system and is subject to other troubles, like plugging of the flow paths. Gas-liquid separation is very difficult for the slurry-state reaction products. In particular, detection of the slurry-state liquid interface under the existing high temperature and pressure would be technically difficult; apparatus for reducing the pressure of the slurry-state liquid reaction product under high temperature and pressure would suffer extreme corrosion and erosion, and would require special technical considerations from the viewpoint of safety and reliability. Stable operation would be difficult because of the contamination of the slurry in the solvent deasphalting step. In the case where the slurry containing a large quantity of asphaltenic material is discharged from the recycling system, the solid-removing procedure is complicated and, moreover, disposal of the discharged material is a problem. A special pump with special reliability and durability is necessary for recycling transportation and boosted feeding of the slurry-state reactants and reaction products, etc.
As is described above, there are still many problems which must be solved before commercializing the above described process.
An object of the present invention is to provide a process for converting a heavy oil into an asphaltene-free and heavy metal-free oil by effective hydrotreating when said heavy oil contains asphaltenic material in such large amounts that it cannot be processed according to the aforesaid conventional processes.